Cut the zucchini in half crosswise. Cut each half into ¼/-inch slices (total 8 slices).
Lay the zucchini slices, mushrooms and red bell pepper on a baking sheet. Brush on both sides with olive oil and season with ground cumin, salt and pepper.
Grill until the vegetables are tender, but not overcooked, about 3 minutes per side for the mushrooms and red bell pepper, and 2 minutes per side for the zucchini. Cut the mushrooms into 8 strips and the bell pepper into 16 strips.
Lightly coat a 9- by 13-inch baking dish with cooking spray. Spread ½ cup of the green enchilada sauce on the bottom of the dish.
Pour 1 ¼ cups enchilada sauce in a shallow dish.
Heat a small skillet over medium heat. Working with one tortilla at a time, heat the tortilla in the skillet for 5 seconds per side to soften. Dip the tortilla into the enchilada sauce in the shallow dish, coating both sides.
In the center of each tortilla, arrange 1 portobello strip, 2 bell pepper strips and 1 zucchini piece. Divide 2 ounces of the queso fresco and 3 tablespoons of the cilantro between the enchiladas.
Roll the enchiladas and arrange, seam side down, in the prepared baking dish.
Spread ½ cup of the enchilada sauce over the enchiladas and top with the remaining queso fresco.
Bake until the sauce is bubbling and the cheese is melted, about 20 minutes.
Garnish with the remaining cilantro. Arrange the tomatoes and avocado over top, if desired. Serve.